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The marine filamentous N2 fixing (diazotroph) cyanobacteria Trichodesmium spp. form extensive blooms contributing 25 to 50% of the estimated rates of N2 fixation in the oligotrophic subtropical and tropical oceans. Trichodesmium’s dominant role in carbon and nitrogen cycling has prompted investigations examining the effects of rising sea surface temperatures and elevated atmospheric pCO2 (leading to ocean acidification) on the growth and abundance of this organism. We examined the effect of elevated pCO2 and light on the physiology and gene expression of key genes in Trichodesmium. Genes that participate in nitrogen metabolism, Ci fixation, and photosynthesis were most affected by changes in pCO2, temperature and the time within the diurnal period. High pCO2 shifted transcript patterns of all genes, resulting in a more synchronized diel expression. Concurrently, we found no significant changes in the protein pools or in total cellular allocations of carbon and nitrogen (i.e. C : N ratio remained stable). Moreover, increased temperatures and high pCO2 resulted in higher C : P ratios. The plasticity in phosphorous stoichiometry combined with higher enzymatic efficiencies lead to higher growth rates. We demonstrate that shifted cellular resource and energy allocation among those components will enable Trichodesmium grown at elevated temperatures and pCO2 to extend its niche in the future ocean, through both tolerance of a broader temperature range and higher P plasticity.

Currently we are investigating two other physiological phenomena in Trichodesmium: the mechanisms of colony and bloom formation, and the molecular mechanisms of toxin production.

Photosynthesis and fixation of atmospheric dinitrogen which are two fundamental processes that are performed by organisms at the basis of aquatic food webs – the phytoplankton and bacterioplankton. Dinitrogen fixation is a biological transformation carried out only by a subgroup of prokaryotic organisms (diazotrophs) that can utilize atmospheric nitrogen (N2), unavailable for most organisms, and convert it into a form of nitrogen that is used for growth. This process is extremely important in many nitrogen-poor surface waters of the oceans, and injects a new source of bioavailable nitrogen to areas where nitrogen limits growth and primary production by the ocean’s tiny plants – the phytoplankton.

In our research we explore how diazotrophs influence bio-geochemical cycling of carbon and nitrogen in the face of climatic changes. These changes include global warming and ocean acidification due to increased dissolution of atmospheric CO2 in the oceans.

We have specifically examined changes in natural populations of diazotrophs from the Red- and the Mediterranean Seas under the combined effects of elevated CO2 and higher temperatures as well as variations in essential nutrients such as phosphate and iron. Collaboration with Dr. Yeala Shaked (IUI- Hebrew University) on iron uptake of Red Sea populations of Trichodesmium (a globally important N2 fixer in the tropical and subtropical oceans with blooms extending over thousands of kilometers) has illustrated that the colonies actively take up and shuttle Fe along the filaments to the center of the colony where it is dissolved and assimilated into the cells (Photo 1). We have also demonstrated that the future projections of high CO2 in the oceans can enhance nitrogen fixation and growth of this marine cyanobacterium and indicate that Trichodesmium will thrive in the future warmer and more acidified oceans (Photo 2).

Further research on other diazotrophs in the Gulf of Eilat shows that a large diversity including bacterioplankton. These are not restricted like the phytoplankton to the upper sunlit areas of the surface oceans, and can fix nitrogen in deep dark layers. We measure N2 fixation rates from oceanic zones that have traditionally been ignored as sources of biological N2 fixation; the dark, fully oxygenated, nitrate (NO3–)-rich, waters of the oligotrophic Gulf of Aqaba and the eastern Mediterranean. Our results suggest that while N2 fixation may be limited in the surface waters of the oligotrophic Mediterranean and Red Seas, N2 fixation from the deeper and dark ocean layers may contribute significantly to new N inputs, yet these inputs are currently not included in regional or global N budgets.

Trichodesmium is a filamentous, non-heterocystous cyanobacteria whose filaments (trichomes) are composed of 10-100s of cells with similar morphologies. Trichomes can be found as single filaments, spherical (“puffs”) or fusiform (“tufts”) colonies . The colonies provide unique habitats for other organisms (metazoans, bacteria, and viruses) and serve as hot-spots for microbial mediated nutrient transformations within the oligotrophic oceans. While this phenomenon is a well-known trait of Trichodesmium the mechanisms for the formation of these structures are not understood.

In my research, I am interested in determining what causes the single filaments to create colonies; to reveal the cues and mechanisms involved in creating colonies from single trichomes. My work will combine live-imaging microscopy as well as molecular and physiological techniques.

My research focuses on the factors which regulate programmed cell death (PCD) in cyanobacteria. Specifically, I study PCD in the marine cyanobacteria Trichodesmium which form extensive blooms in the tropical and subtropical surface-oceans. Trichodesmium undergoes an autocatalytic, genetically programmed cell death in response to environmental stressors such as high irradiance and Fe limitation. They have key enzymes of the eukaryotic PCD and several proteins from the metacaspase family. I study the role of PCD in blooms and cell death mechanism in Trichodesmium. My work involves both laboratory experiments with Trichodesmium cultures, and field experiments with natural Trichodesmium blooms. Molecular and physiological approaches are applied to examine expression of PCD in Trichodesmium at both the genetic and protein levels.

E-mail: dina.spungin@gmail.com

Trichodesmium bloom in the South West Pacific Ocean (New Caledonia).Photo by D. Spungin

The ocean is a significant sink of anthropogenic CO2, in large part because organic matter is exported to oceanic depths driving the biological sequestration of carbon in the ocean’s interior. Organic matter export depends on the supply of external nutrients to the euphotic zone (by processes such as deep mixing and biological fixation of atmospheric dinitrogen (N2)) and the subsequent production of organic matter by photosynthesis (defined as “new” production).

The Gulf of Aqaba offers a unique opportunity to observe, at high temporal resolution, both mechanisms supplying “new” nutrients. The oligotrophic Gulf is surrounded by land on three sides and characterized by a thermohaline circulation pattern caused by high evaporation rates, and exhibits strong seasonal variability mainly due to deep winter mixing and strong summer stratification.

My main goal is to understand the relative contribution of N2 fixation and deep winter mixing to “new” production and subsequent “export” in the northern Gulf of Aqaba and to simulate and predict the response of the system under changing environmental scenarios.

Biological fixation of atmospheric dinitrogen can contribute significant amounts of biologically available dinitrogen to the nitrogen-limited surface waters of the oceans and induce subsequent biological production

The marine filamentous N2 fixing (diazotroph) cyanobacteria Trichodesmium spp. form extensive blooms contributing 25 to 50% of the estimated rates of N2 fixation in the oligotrophic subtropical and tropical oceans. Trichodesmium’s dominant role in carbon and nitrogen cycling has prompted investigations examining the effects of rising sea surface temperatures and elevated atmospheric pCO2 (leading to ocean acidification) on the growth and abundance of this organism.

We examined combined effects of elevated pCO2 , changes in light and nutrients on the physiology and gene expression of key genes in Trichodesmium. Genes that participate in nitrogen metabolism, Ci fixation, and photosynthesis were most affected by changes in pCO2, temperature and the time within the diurnal period. High pCO2 shifted transcript patterns of all genes, resulting in a more synchronized diel expression. Concurrently, we found no significant changes in the protein pools or in total cellular allocations of carbon and nitrogen (i.e. C : N ratio remained stable). Moreover, increased temperatures and high pCO2 resulted in higher C : P ratios. The plasticity in phosphorous stoichiometry combined with higher enzymatic efficiencies lead to higher growth rates. We demonstrate that shifted cellular resource and energy allocation among those components will enable Trichodesmium grown at elevated temperatures and pCO2 to extend its niche in the future ocean, through both tolerance of a broader temperature range and higher P plasticity.

Currently we are investigating two other physiological phenomena in Trichodesmium: the mechanisms of colony and bloom formation, and the molecular mechanisms of toxin production.

Programmed cell death (PCD) is an irreversible, genetically controlled form of cell suicide that is essential to promote and maintain genetic stability and is critical for the regulation of cellular and tissue homeostasis in metazoans. PCD has been observed in a variety of unicellular organisms including prokaryotic bloom-forming cyanobacteria, chlorophytes, dinoflagellates, diatoms, and coccolithophores. Trichodesmium also displays autocatalytic PCD in response to stressors such as oxidation, high irradiance, and Fe-depletion. In the oceans Trichodesmium forms extensive blooms in nutrient-poor tropical and subtropical regions. These massive blooms generally collapse several days after forming, but the cellular mechanism responsible along with the magnitude of associated C and N export, are as yet unknown. Our work from laboratory simulations demonstrate that extremely rapid development and abrupt, PCD-induced demise (within 2-3 d) of Trichodesmium blooms lead to greatly elevated excretions of transparent exopolymers and a massive downward pulse of particulate organic matter. Our results mechanistically link autocatalytic PCD and bloom collapse to quantitative C and N export fluxes and suggest that PCD may impact biological pump efficiency in the oceans.